Avalanche photo-diodes (APDs) have been used as electromagnetic radiation (EMR) detectors, and have applications, such as for single photon counting experiments, imaging and telecommunications.
There are different styles of APDs, including ‘reach-through’ (RAPD), separate absorption-multiplication regions (SAM), separate absorption grading and multiplication (SAGM)—and super-lattice stair-case band-gap designs, which are admixtures of these concepts. For all of these APD designs, incident EMR is absorbed as much as possible in some semiconductor region (absorption region), and the resulting electrons are then fed efficiently to a p-n junction region (multiplication region) where avalanche multiplication occurs, either just above or just below the Geiger-breakdown ‘knee-’ characteristic of the APD design.
FIG. 1 schematically illustrates a typical separate absorption region 10 and multiplication region 20 of an APD structure known as a SAM-APD. In FIG. 1, photons get absorbed in an n− or π-region, the absorption region 10, and the resulting photoelectrons are then multiplied in the p-n interface region, i.e., the multiplication region 20. Schottky-barrier APDs with n-type silicon substrates are particularly useful for high-speed detectors and for detection of ultra-violet (UV) light. Normally the UV light is transmitted through a thin metal layer and absorbed in the first 10 nm or so of the silicon. The carrier multiplication is then mainly due to electrons, resulting in low noise. Photo-excitation over the barrier extends the wavelength-range beyond that of the band-gap. FIG. 2 illustrates an APD with a relatively thick π absorption region of about 50 microns.
More sophisticated and complicated device structures have been developed with the intent of reducing the k value, the ionization rate ratio of holes to electrons, of the APD to be as small as possible to reduce the noise level of the device, and increase its useful performance. FIG. 3 illustrates a super-lattice APD based on III-V semiconductor materials, such as GaAs, with a performance close to silicon, which is a group IV semiconductor material. FIG. 4A illustrates a separate absorption charge multiplication structure (SACM-APD) which permits different electric fields in the absorption and multiplication regions, while FIG. 4B illustrates a SAM-APD. With traditional p-n junction APD devices and absorption processes, the operating temperature substantially affects both the noise level and bias voltage point of the Geiger breakdown in APDs.
What is needed is an APD device with low k values to allow for high photo-level detection performance, even at room temperature, for operation in the visible (VIS) and near infrared (NIR) wavelength regimes. The absence of the need for cooling the detector while maintaining high performance would allow for a device with very low weight and power consumption, and would provide for a detector for applications in the lowest light level environments, such as heavily overcast starlight at night.